This invention relates to engines powered by the burning of fuel in air or other oxidant and, more particularly, to the electronic control of the air-fuel ratio.
The internal combustion engine is commonly used for driving a large variety of vehicles and machinery. The engines may burn hydrocarbon fuels in gaseous or liquid form. The products of combustion, water, unburned hydrocarbons, oxides of carbon and oxides of nitrogen, vary in their respective concentration depending in part upon the air-fuel ratio at the input of the engine. Also, the efficiency of the engine is dependent on the air-fuel ratio. Accordingly, in many situations it is important to control the air-fuel ratio as a function of at least one output gas such as oxygen which has not combined with the fuel so as to provide for desired levels of engine emissions and efficiency.
One form of electronic control commonly in use comprises a feedback circuit in which an air-fuel control mixture system or means such as a mixing valve is operated in response to the concentration of exhaust oxygen. The oxygen is frequently sensed using a solid state electrochemical cell employing zirconia as the electrolyte. Such a zirconia probe produces an electric voltage in the range of approximately 30 mv-1000 mv (millivolts) dependent on the concentration of oxygen in the exhaust gases. The accuracy of the air-fuel control is therefore dependent on the accuracy of the voltage produced by the zirconia sensor relative to the air-fuel ratio.
A problem arises in that the characteristic sensor output curve is influenced by aging of the zirconia sensor due to conditions in the exhaust as well as being dependent upon temperature conditions. Reference is had to the Society of Automotive Engineer's technical paper 800017 entitled "Three Years Field Experience with the Lambda-Sensor In Automotive Control Systems" published on Feb. 25, 1980. Thus, a control system which uses a predetermined set point voltage for control of a specific air-fuel ratio would later provide a different air-fuel ratio for the same set point voltage due to a shift of the characteristic output curve.
As an example in control systems utilizing the sensing of exhaust emissions as a part of a feedback loop, the following is of interest.
U.S. Pat. No. 4,120,269 which issued in the name of Fujishiro on Oct. 17, 1978 discloses in FIG. 3 a reference signal taken as a ratio of a voltage stored across the capacitor in the compensation of a zirconia probe.
U.S. Pat. No. 4,131,089 which issued in the name of Fujishiro et al on Dec. 26, 1978 discloses in FIG. 4 and in column 4 a limitation on the swing of a reference voltage for compensation in characteristics of a zirconia probe.
U.S. Pat. No. 4,142,482 which issued in the name of Asano et al on Mar. 6, 1979 similarly shows a circuit (item 12 in FIGS. 1 and 4) for the limitation on the swing of a reference voltage in the compensation for shift in an automotive exhaust sensor.
U.S. Pat. No. 4,167,925 which issued in the name of Hosaka et al on Sept. 18, 1979 employs circuitry for the compensation of variation in the gas sensor based on maximum swings in the sensor voltage as disclosed in FIGS. 3 and 4.
U.S. Pat. No. 4,170,965 which issued in the name of Aono on Oct. 16, 1979 discloses a mean value circuit (FIG. 4 and Column 4) wherein a capacitor stores a mean value of exhaust sensor, a ratio circuit coupled thereto providing a reference signal for use in compensation in exhaust sensor.
U.S. Pat. No. 4,203,394 which issued in the name of Anono et al on May 29, 1980 discloses an averaging circuit (item 18 in FIG. 2 and bottom of Column 2) to compensate for fluctuations in sensor output.
The above patents disclose emission control systems which rely on controlled perturbations or oscillations of the air-fuel ratio. The present invention does not have nor require such perturbations. Pollutants, such as CO and especially NO.sub.x are easier to control in the present invention. This is particularly true in a steady-state, lower RPM engine operating environment in a non-perturbating system.
In addition, the following U.S. patents are of general interest in this area: U.S. Pat. Nos. 4,177,770; 4,177,787; 4,121,548; 4,117,815; 4,019,474; and 3,984,976. Reference is also made to a U.S. copending patent application assigned to the same assignee as this application entitled "Method and Means For Controlling Air-to-Fuel Ratio", By Kenneth R. Burns and John J. Early; Ser. No. 433,199; filed on Oct. 7, 1982. This copending application and the other patents and publications cited herein are incorporated by reference in their entirety in this application.